Fifty years ago, tornadoes in the United States were most common over the Great Plains. A bull’s-eye of sorts covered parts of Texas, Oklahoma, Kansas and Nebraska. The term “Tornado Alley” entered American lexicon.

But in the years since, that hot spot of tornadoes has shifted markedly east. Researchers have pointed to parts of the Deep South and Tennessee Valley as being the modern-day Tornado Alley.

A study published in the Journal of Applied Meteorology and Climatology in April confirms the legitimacy of this shift and highlights a change in when tornadoes are likely to occur.

The results spell problems for residents of the Midwest and Southeast, where a higher population density and a greater prevalence of mobile homes leads to a more serious vulnerability to tornadoes.

Experts still aren’t sure exactly why tornado activity has moved - and whether it’s associated with long-term atmospheric-ocean cycles, human-caused climate change or the result of unknown processes.

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What the study did

To evaluate trends in tornado activity, researchers examined tornado reports from National Weather Service archives. They analyzed only tornadoes rated 1 or higher on the 0-to-5 scale for intensity, because it’s believed that tornadoes rated 0 are being detected more frequently now.

The researchers then divided the nation into grid boxes and looked at changes in tornado activity between two time periods, each spanning 35 years. The first ran from 1951 to 1985, and the second from 1986 to 2020.

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What the researchers found

Several key findings emerged from the study, all of which point to an increase in tornado risk across the South and decrease in the Plains.

- The largest decrease in tornadoes between the two periods was in eastern Kansas through Oklahoma and north Texas. A 40 percent reduction was noted. The greatest increase in tornadoes was in southern Mississippi, with a 25 percent uptick.

- With respect to how many days a year tornadoes touched down within a given grid box, the bull’s eye during the first period was on south central Oklahoma. Within the more recent period, that jackpot zone shifted to southern Mississippi.

- A large zone from Texas to Nebraska experienced fewer tornado days, but southern Mississippi, Middle Tennessee and western Kentucky saw an increase. There was also an increase in tornado days in the Mid-Atlantic, including central and eastern Maryland, where a historic outbreak occurred Wednesday.

- In terms of path length, or distance covered by tornadoes, similar trends were observed. The study found a decrease in total path length over the Great Plains and an increase in parts of the Midwest and Southeast.

- For significant tornadoes rated 2 out of 5 or higher, the trends are even more robust. There’s a marked decrease over the Great Plains, and an increase for the Deep South and Tennessee Valley, especially in Middle/western Tennessee and western Kentucky.

- In general, researchers found a decrease in tornado activity during summer and May, which has historically been the busiest month for tornadoes. Cool-season tornadoes are becoming common, especially across the Midwest and Southeast, particularly from November to March.

Researchers not involved in the study said the reported trends are convincing but pointed out some limitations in the analysis.

Karen Kosiba, a tornado researcher at the Center for Severe Weather Research, said the greater population density in the Southeast compared to the Plains exposes it to more damage. That may be part of why twisters in Alabama and Mississippi appear to be on the ground for longer compared to their Plains counterparts.

The increased population density “increases reporting, but also damage potential … and impact … and path length reports,” she said in an email.

Grady Dixon, a professor and researcher specializing in tornado trends at Fort Hays State University, shared a similar perspective. He said he wasn’t sure how much of the increase in tornado activity in the east is real.

“The decrease in the west is almost certainly a real signal,” Dixon said. “However, I think it is possible that activity in the east has been rather steady [over time], but I do not know how we could confirm that point.”

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What the results mean

The study reaffirms something that social scientists and meteorologists alike have been worried about: The South, where tornado vulnerability is particularly high because of its dense population and concentration of mobile homes, may now be subject to an even greater risk.

But it’s still not evident why the shift toward more tornado activity in this area is happening. El Niño and La Niña, for example, can affect how busy a particular season is, but their influence is washed out over a period of decades.

It remains unclear whether this apparent eastward shift in tornado incidence is something that will reverse or become more pronounced.

“If climate change is driving the changes in both regions, then we should expect the shift/changes to be rather permanent … or at least long-lasting,” Dixon said.

But if the changes are part of a cycle, they should eventually change course.

Researchers hesitate to say what will happen next.

“[A] big question is if [the shift] is ‘permanent’ … or just another example of variability,” Harold Brooks, a researcher at the National Severe Storms Laboratory, said in an email. “We don’t have any good understanding of reasons for the shift.”

Victor Gensini, a leading researcher on tornado trends in the United States, isn’t persuaded that the shift is enduring.

“I do not agree with the idea of a ‘shifting’ frequency,” he wrote in an email. “If it ‘shifted,’ why did we have so many tornadoes in the Great Plains this year?”

Because twister hot spots jump around so much, Gensini and other researchers have expressed a general dislike for the term “Tornado Alley.”

“We need to stop using tornado alley to describe a specific region,” he said. “The reality is that tornadoes can occur in all 50 states on any calendar day if the ingredients are present.”

This year, for example, the Plains experienced an unusually busy tornado period in late April and May, whereas most of Mississippi and Alabama were unusually quiet.

So even if the nation’s most extreme tornado hot spot has moved east, away from the Plains, “there will still be days or years … when [it’s] a prolific producer of tornadoes,” Dixon said.